Neither trinket is particularly good for shadow priests: both have 2 minute cooldowns, which don't sync up with any of our talented cooldowns (Dark Archangel has a 90 sec CD). If you can find a BoT pug, Theralion's Mirror is probably your best non-Firelands option.

Anyway, I would expect Moonwell Chalice to beat out Soul Casket by a pretty wide margin, but if you really must have a numerical answer, use SimulationCraft and/or AskMrRobot.